Magnesium is a useful and valuable metal and is commonly used in aluminium alloys, in die-casting (alloyed with zinc), to remove sulfur in the production of iron and steel, and in the production of titanium. Magnesium is used in several high volume part manufacturing applications, including automotive and machine components. Because of its low weight, good mechanical and electrical properties, magnesium is widely used for manufacturing of mobile phones, laptop computers, cameras, and other electronic components.
Most of the world supply of magnesium comes from processing naturally occurring materials such as dolomite and magnesite. Another potential source of magnesium is waste ash material from coal fired power stations burning brown coals.
There are several brown coal deposits around the world. Some of the larger deposits of brown coal, also referred to as lignite, are found in Russia, the United States Germany, Poland and Australia.
The brown coal is typically prepared as a pulverized fine powder (PF) in which form it is delivered to vertical water wall boilers where it is combusted to release heat for steam generation by turbines. The majority of the combustion products are fine particles which are carried by the flue gases out of the boiler and are known as fly ash. The coarser ash particles, principally sand, settle to the bottom of the boiler from where they are collected. This fraction is known as bottom ash and generally constitutes about 20% of the total ash content of the combusted coal. The flue gases from the boiler are often treated with an electrostatic precipitator to remove the fine particles (>99%) and this fraction is known as electrostatic precipitator (EP) fly ash and comprises about 80% of the total ash content of the combusted coal. The fly ash typically contains about 5-20% char (unburnt or partially carbonised coal).
The two ash types are typically mixed with recycled ash pond water and temporarily held in a large ash pit within the power station where some chemical reactions and hydrochemical alterations to the ash begin to occur. The mixed ash slurry, with a liquid to solid ratio typically ranging from about 100:1 down to 10:1, depending on the particular power station, is then pumped to an ash pond for disposal.
Emplaced ash typically continues to ‘age’, i.e. undergoes further chemical alterations, including most notably hydration and decomposition of brownmillerite (calcium alumino-ferrite) Ca2(Al,Fe)2O5 to a variety of products such as hydrated calcium alumino-ferrites, hematite, iron hydrotalcites, hematite and magnetite and absorption of carbon dioxide (CO2) from the atmosphere which may markedly increase the chemically fixed CO2 content of the emplaced mixed ash. The CO2 content of raw dry EP fly ash is relatively low, typically <1.0% but with ageing in a wet or dry ash emplacement the emplaced mixed ash CO2 content slowly increases, with the CO2 being largely fixed as calcite (calcium carbonate; CaCO3) and magnesite (magnesium carbonate; MgCO3).
The ash from brown coals has a wide variety of applications including soil conditioning/fertilization, as an extender in cement and concrete production and as fillers in non-metallic minerals and building materials. The relatively high magnesium and calcium contents in brown coals results in the brown coal fly ash being classified as ‘Class C’ fly ash in the American classification system and this also raises the possibility of recovering magnesium (Mg) from the fly ash.
One of the principal methods of manufacture of magnesium metal from suitable feedstock is the pyrometallurgical method known as the Pidgeon Process.
Most raw materials collected from brown coal power generation ash pits or ponds do not have suitable compositional qualities for direct conversion to magnesium using the Pidgeon Process, Failure to treat such raw material, as well as other raw material having similar composition, to achieve compositional qualities suited to the Pidgeon Process or other suitable reductive pyrometallurgical process may inhibit or prevent magnesium formation. Furthermore, magnesium generated from the raw starting material may have multiple impurities rendering it unfit for commercial use or sale.
Furthermore, the calcining stage typically used in the Pidgeon Process to convert dolomite-type feedstock into dame-type form for generation of magnesium requires significant temperatures, which can be energy and cost inefficient.
The present inventor has developed a process particularly suitable for processing fly ash and other materials for reductive pyrometallurgical magnesium production by the Pidgeon Process or other suitable reductive pyrometallurgical processes.